What Those Fukushima Radiation Counts Really Mean: Analysis

Radiation safety expert Andrew Karam has written for Popular Mechanics about what went wrong at Fukushima and how Japan reacted to the emergency. Now he's just back from a trip to Japan, where he took radiation detectors to assess the contamination levels himself. Not all radiation measurements are created equal, so Karam walks us through what the numbers, and the different radiation units, really mean.

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As anyone who follows the crisis at the Fukushima nuclear plant knows, there have been a number of radiation statistics, expressed in various units, coming from different sources—including organizations like the International Atomic Energy Agency (IAEA), which released regular updates of radiation levels. The IAEA made it clear that levels further away from the reactor site are not dangerously high, but as a scientist and a former Navy nuclear power technician, I was curious to learn more firsthand. So I went to Japan, where I traveled within about 12 miles of the reactors, and spent nearly two weeks taking my own measurements using several different tools.

When it comes to human health, radiation-safety professionals are concerned with more than just overall radiation output. First, we care about radiation dose: the amount of radioactive energy deposited in an object or person. This is measured in rads in the U.S. and in Grays in the SI unit system.

Secondly, we worry about the kind of radiation. Just as a bowling ball causes more damage than a tennis ball, some types of radiation (alpha particles or neutrons, for example) cause more biological damage than others (such as beta or gamma radiation). The more dangerous types of radiation have a higher relative biological effectiveness (RBE) level. The unit of measure called rem or millirem (or Sievert in SI units), which you've probably heard mentioned during coverage of Fukushima, takes RBE into account. Therefore, rem measures not just radiation dose, but also the biological harm that radiation can cause. Most radiation workers are limited to 2000 millirem (mrem) per year and up to 25,000 mrem during emergencies; we begin to see radiation sickness at 100,000 mrem. By comparison, most of us receive about 300 mrem annually from natural radiation, and a single whole-body CT scan will give us about 2000 mrem. In the areas of Japan I visited, radiation dose rates were elevated to about three to four times typical natural radiation dose rates (which are about .1 mrem per hour), but nowhere near as high as natural radiation levels I've measured in parts of Iran.

Finally, radiation experts also survey how much radioactive contamination a person or object has received, measured in counts per minute (cpm) or in decays per minute (dpm). (Contamination is when radioactivity appears where it's not wanted—sort of like dust.) Despite the doom and gloom associated with the term in popular parlance, however, what radiation safety experts are referring to when we say "contamination" is rarely harmful. Because it's a measure of how much radiation had accumulated on you, it goes down when you clean up. I think of it like changing a diaper—I don't want to get contaminated, but if I do, I just wash it off and go on with my day. Japanese officials are surveying people leaving the affected areas, and anyone with more than 100,000 cpm of contamination is asked to change clothes (which removes up to 90 percent of the contamination) and possibly to shower. As soon as their contamination levels are below this number, they're approved to go.

So with all these different kinds of data, what gear do you need to make the right measurements? There are two fundamental types of radiation instruments—those that measure only the number of times radiation interacts with the detector, and those that measure the amount of energy deposited. The Geiger-Mueller tube (the Geiger counter), probably the most widely known radiation instrument, falls into the first category. Geiger counters can measure alpha, beta and gamma radiation, making them the most versatile kind of radiation detector, but they are typically used to measure contamination only. That's because a normal Geiger counter can't distinguish between high- and low-energy radiation; everything looks the same to it. It's like holding a handful of rocks, pebbles and grains of sand and simply counting every object: A single grain of sand counts the same as a rock, even though we know that rocks are larger and weigh more. So Geiger counters are great for measuring contamination, but not so great at measuring radiation dose rate, the biological part of the equation.

Getting at the radiation dose rate requires measuring the amount of energy deposited by the radiation and, therefore, requires different equipment. An ion chamber measures the energy deposited in a mass of air by measuring the amount of electrical charge (which is related to ionization) that occurs in the chamber. Ionization chambers can directly measure radiation dose rate to a fairly high degree of accuracy.

Another way to measure radiation dose rate is to use a device called a scintillation counter—a detector that uses a crystal, such as sodium iodide. When radiation interacts with the crystal, the crystal emits light, and the amount of light emitted is proportional to the energy deposited in the crystal (the radiation dose). Scintillation detectors can measure both count rate (contamination) and dose rate. The drawback is that scintillation counters can usually measure only one type of radiation (alpha, beta or, in the case of sodium iodide, gamma). But they measure it very well. And though Geiger counters aren't intended for radiation dose rate measurements, they can do it in special circumstances if the other detectors aren't available. One kind of Geiger counter—called an energy-compensated GM—adjusts for different energies of radiation. If one of those is not around, a conversion chart can correct what the Geiger counter says to what the dose rate really is.

In Japan I had a number of instruments with me. One is a type of scintillation counter that's set up to identify specific radionuclides by measuring the energy of the gamma ray photons it sees—a process called gamma spectroscopy. This way I was able to identify long-lasting isotopes of cesium, like Cs-134 and Cs-137, that came from Fukushima. But nearly all of the radioactive iodine—which had been a concern early on—has decayed away by now, so I could not detect any of it. I had a pressurized ion chamber to measure radiation dose rate at the low levels of exposure detected in Japan. I also carried a regular ion chamber and an energy-compensated GM in case dose rates become higher than what the pressurized ion chamber will read, but we were not exposed to any level of radiation high enough to need either one. (My total radiation exposure—12 millirem—is almost entirely due to air travel.) Finally, I've always got my Instadose—a device that I can plug into my laptop computer to read my dose directly. I have to admit, carrying all of these instruments around started to wear on my shoulder (and explaining them to airport security was a challenge). But the information they provide is interesting enough to make the bother worthwhile.

Andrew Karam has over 30 years of experience in health physics (radiation safety), beginning with an eight-year stint as a mechanical operator and radiation safety specialist in the Navy. Since then, Karam has worked for the State of Ohio, the Ohio State University, the University of Rochester and as a private consultant. He is the author of five books and an upcoming eight-part series, Controversies in Science. He currently lives in New York City, where he works on issues related to our response to radiological and nuclear emergencies. Check out his website.